1. Field of the Invention
This invention relates to current injection type semiconductor optical devices with improved leakage current characteristics.
2. Description of the Prior Art
A current injection type semiconductor optical devices such as, for example, a semiconductor laser diode which can lase at a low threshold current in fundamental lateral mode, a semiconductor laser optical amplifier have current blocking layers embedded at both sides of a mesa stripe including an active layer (a light emitting region) to inject and constrict current in the active layer.
As such a current blocking layer, a structure including a pn junction is generally employed.
FIG. 6 shows a known semiconductor laser diode manufactured through processes including a liquid-phase epitaxial method (LPE method).
In the case of the semiconductor laser diode shown in FIG. 6, an n-InP clad layer 2, a GaInAsP active layer 3, p-InP clad layers 4a, 4b, a p-InP current blocking layer 6, an n-InP current confining layer 7, and a GaInAsP cap layer 5 are formed on a predetermined portion of an n-InP substrate 1, and an n-electrode 8 is formed on the lower surface of the substrate 1, and a p-electrode 9 is provided on the upper surface of the cap layer 5.
In the diode structure of FIG. 6, the pn junction comprising the p-InP current blocking layer and the n-InP clad layer 2 is forward biased similarly to the pn double heterojunction including active layer 3 under lasing conditions, and the p-InP clad layer 4b, the n-InP current confining layer 7, the p-InP current blocking layer and the n-Inp clad layer 2 constitute a pnpn (InP) thyristor structure 10 as indicated by broken lines in FIG. 6.
In the case of the semiconductor laser diode manufactured as described above, as shown in FIG. 4, most current is injected into the region of the active layer 3 due to the difference of built-in potentials i.e. forward bias voltages required for current rising up, but a leakage current through the pass I to II in FIG. 6 is also generated to become a gate current of the thyristor structure 10, as the pn junction comprising the current blocking layer 6 and clad layer 2 is also forward biased as described above. Therefore, an anode current of the thyristor structure 10 also flows as a leakage current through the pass III to IV in FIG. 6.
FIG. 7 shows a semiconductor laser diode by an MOCVD method.
In the case of the semiconductor laser diode in FIG. 7, the diode has the same arrangement as the semiconductor laser diode of FIG. 6, and a p-InP clad layer 4b, an n-InP layer 7, a p-InP layer 6, and an n-InP clad layer 2 are formed in a pnpn (InP) thyristor structure 10.
In the step of forming a current blocking layer in the semiconductor laser diode of FIG. 7, when the p-InP layer 6 (the current blocking layer) is grown by an MOCVD method, the initial layer of the p-InP layer 6 is so grown as to rise along the side surface of a mesa stripe as its peculiar phenomenon of this method, and the contacting area of the p-InP clad layer 6 and the p-InP layer 6 is generated.
The larger the contacting area of both the layers 4a and 6 is larger, the larger the leakage current which flows in a direction from I to an arrow II in FIG. 7 flow. In addition, since this corresponds to the gate current of the thyristor structure 10, the anode current of the thyristor structure 10 becomes larger, and current blocking characteristic is more deteriorated.
As a present remedy, the p-InP clad layer 4a is reduced in thickness as the contacting area is proportional to the thickness of the p-InP clad layer 4a and the p-InP layer 6 is increased in resistance thereby enhancing the current blocking characteristic of the semiconductor laser diode prepared by the MOCVD method in the same degree as that of semiconductor laser diode by the LPE method.
In addition, when mainly by an MOCVD is employed as a manufacturing method, a semiconductor laser diode shown in FIG. 8 can be also manufactured.
In the semiconductor laser diode of FIG. 8, a p-InP clad layer 12, a GaInAsP active layer 13, n-InP clad layer 14a, 14b, an n-GaInAsP cap layer 15, an n-InP current confining layer 16, a p-InP current blocking layer 17 are formed on a p-InP substrate 11, a p-electrode 9 is formed on the lower surface of the substrate 11, and an n-electrode 8 is formed on the upper surface of the cap layer 15.